Firetubes are used to heat a fluid by allowing heat transfer from hot gases from a burner to pass through a sealed chamber which is immersed in a liquid. The heat of the gas is transferred through the walls of the firetube by thermal conduction, and then heating the liquid, usually to its boiling point so that it transitions into a gas.
One known type of firetube has a cylindrical chamber with fins secured to the interior surface of the cylinder to increase the internal surface area. The firetube is surrounded by fluid, which is heated through contact with the outer surface of the cylinder. Combustive products, such as from natural gas or propane, exiting a burner head, enter the firetube and a portion of the resultant hot gas is funneled to flow outside of the fins and a portion is funneled to flow between the fins. The hot gas heats the fins from both sides and also heats the cylinder. The fins, being in contact with the cylinder, transfer their heat to the cylinder, and the cylinder heats the fluid around the firetube. A core plug within the cylinder forces the hot gas to flow near to the fins for a higher heat transfer coefficient. This provides a large heat output in a relatively short, small diameter tube.
Firetubes can be employed in continuous-cycle absorption cooling systems for the purpose of vaporizing ammonia in a water-ammonia solution. Such ammonia-based cooling systems are commonly used for air conditioning and refrigeration. These systems typically contain a generator, a condenser, an evaporator, and an absorber, with firetubes being one type of generator system employed. The system is filled with ammonia and water, at sufficient pressure for ammonia to condense into a liquid at operating temperature.
In such systems, water-ammonia solution is heated by a firetube. This heating produces bubbles of ammonia gas and water vapor which are then passed through a rectifier. In the rectifier, condensate for generator reflux is produced while nearly pure ammonia vapor can pass to a condenser where it is cooled and condenses into liquid ammonia. From the condenser the liquid ammonia flows into a subcooler for heat recovery and then through an expansion device for reduction in pressure. From there, the two phase, but mostly liquid, and nearly pure ammonia enters an evaporator, where heat transfer from a chilled heat transfer fluid, such as water or a brine, causes the ammonia to evaporate. The evaporation of the ammonia causes the desired cooling or refrigeration. Next, ammonia gas mixture flows into the absorber, where it comes into contact with the weak water-ammonia solution which flows back to the subcooler and then into the absorber from the generator. In the absorber, the ammonia is absorbed out of the gas mixture into the weak water-ammonia solution, resulting in a strong water-ammonia solution that flows to the generator system, completing the cycle of operation.